The environmentalist questions whether politicians understand the urgency of climate change. We also talk to Ann Druyan, widow of Carl Sagan, about Cosmos - possibly the greatest science documentary series ever made (and you can win a copy). Finally, a visit to the Science Museum's new exhibition on the cultural impact of astronomy

As the founding director of the sustainable development NGO, Forum for the Future, and, until this month, chair of the UK Sustainable Development Commission, when Porritt speaks about global warming people listen. The former director of Friends of the Earth and trustee of WWF came into the pod to fill in the British government's scorecard on tackling climate change.

The astronomer Carl Sagan was a prolific scientist, pioneering the study of exobiology and astrochemistry and promoting the search for extraterrestrial life. One of his biggest achievements was Cosmos, a 13-part science documentary series first aired in the US in 1980. In it, he took viewers on a journey around the universe describing everything from atoms to galaxies and set a gold standard for science on television.

Alok Jha speaks to Sagan's widow Ann Druyan, who was also one of the writers on Cosmos.

Pursuing the cosmic theme, we visit a new exhibition at London's Science Museum that shows how astronomy has influenced culture, and how it has changed our behaviour and been popularised. Exhibits include Astronomy Monopoly and a telescope built from baked-bean cans, spare car parts and coat hangers.